1. Field of Invention
The present invention relates to capacitors, especially a method that improves the stability and reliability of the solid electrolytic capacitor.
2. Related Art
Since conducting polymers have a higher conductibility than liquid electrolyte fluid or solid organic semi-conducting salt (such as TCNQ complex salt) that are used in the traditional electrolytic capacitor. Therefore, conducting polymers used as solid electrolyte in the electrolytic capacitors can improve the characteristics of electrolytic capacitors, for example, high-frequency response characteristics.
Jesse S. Shaffer et al. first used conducting polymers as electrolyte in the electrolytic capacitor in U.S. Pat. No. 4,609,971 in 1983. The procedure involves dissolving soluble polyaniline powder and LiClO4 into the mixture solution of butyrolactone and water. And then an anode aluminum foil is dipped into the fore-said mixed solution and expels the solvent on the aluminum foil. Since conducting polymers such as polyaniline cannot sink into the holes on the dielectric layer very well, the capacitors show low capacitance and high impedance. Therefore, Tsuchiya et al. in U.S. Pat. No. 4,864,472 suggested forming manganese dioxide layer on the surface of the dielectric layer in the capacitor and then forming a layer of conducting polymers through electrochemical process.
In addition, Gerhard Hellwig, Stegne and his coworkers documented in U.S. Pat. No. 4,803,596 utilizing conductive polymers as the electrolyte for capacitors by chemical oxidative polymerization. This method dips an anode aluminum foil into monomers and oxidant, respestively. And then the monomers are polymerized under the appropriate conditions. The procedure is accomplished repeatedly to accumulate electrolyte of conducting polymers.
Whether by electrochemical or chemical oxidative polymerization, the produced conducting polymers generally show poor mechanical strength; especially the conducting polymers generated by chemical oxidative polymerization are loose and brittle. The poor mechanical strength of conducting polymer as the electrolyte of capacitors may cause the failure of capacitors, because the conducting polymer in the capacitors are easily broken by outside force during manufacturing, transporting and the usage of the capacitors. Also, the loose structure of conducting polymers implies that the structure stability of conducting polymers at higher temperature is poor. Therefore, after long usage, the conducting polymers can easily separate from the anode and cathode of the capacitor and the capacitor fails finally.
Therefore, improving mechanical strength and structure stability of the polymer electrolyte is an important issue for the development of a solid electrolyte capacitor.
Japan Patent 2001250742 discloses that glass fiber is added into the conducting polymers in the solid electrolyte capacitors to improve the mechanical strength of the electrolyte layer of the capacitor. However, the interface strength between glass fiber and conducting polymer is poor, so the effect of glass fiber strengthening the conducting polymer is not observed. Further more, adding insulated glass fiber into conducting polymer results in the decline of the bulk conductivity of the solid electrolyte.
Europe Patent 0617442 discloses a kind of composite conducting polymer electrolyte. The inventors suggested adding polymer with functional group of carboxylic acid or hydroxyl to electrolytic solution containing conducting polymer monomer and electrolyte and then the linear polymer combined conducting polymer into the said composite conducting polymer, when conductive polymer monomer converts into conducting polymer by electrochemical synthesis. The said composite conducting polymer electrolyte can be attached to the anode surface of the capacitor very well, and has good physical properties, so the reliability of the capacitor is promoted. However, this method can only use soluble linear polymers, and the thermal stability of linear polymer blend of conductive conjugative polymers is lower than the cross-linked polymers. At the same time, before adopting this method, the compatibility between linear polymers and conducting polymer needs to be concerned, to avoid phase separation in high temperature.